which relates fall velocity in a concentrated grain suspension to that for an 

 isolated grain. 



The concentration dependence of fall velocity is important to vertical 

 variations of grain concentrations (Lavelle and Thacker, 1978), to grain 

 suspension processes, and to the fall of a bulk, sediment sample in a settling 

 tube (Ch. 4, Sec. II, 7, b). 



2. Sand and Gravel . 



By definition, the wjrd sand refers to a size class of material, but sand 

 also implies the particular composition, usually quartz (silica). 



In tropical climates, calcium carbonate, especially shell material, is 

 often the dominant material in beach sand. In temperate climates, quartz and 

 feldspar grains are the most abundant, commonly accounting for about 90 

 percent of beach sand (Krumbein and Sloss, 1963, p. 134). 



Because of its resistance to physical and chemical changes and its common 

 occurrence in terrestrial rocks, quartz is the most common mineral found in 

 littoral materials. Durability of littoral materials (resistance to abrasion, 

 crushing, and solution) is usually not a factor within the lifetime of an 

 engineering project (Kuenen, 1956; Rusnak, Stockman, and Hofmann, 1966; Thiel, 

 1940) . Possible exceptions may include basaltic sands on Hawaiian beaches 

 (Moberly, 1968), some fragile carbonate sands which may be crushed to finer 

 sizes when subject to traffic (Duane and Meisburger, 1969, p. 44), and 

 carbonate sands which may be soluable under some conditions (Bricker, 1971). 

 In general, recent information lends further support to the conclusion of 

 Mason (1942) that, "On sandy beaches the loss of material ascribable to 

 abrasion. . .occurs at rates so low as to be of no practical importance in shore 

 protection problems." 



The relative abundance of nonquartz materials is a function of the 

 relative importance of the sources supplying the littoral zone and the 

 materials available at those sources. The small amount of heavy minerals 

 (specific gravity greater than 2.87) usually found in sand samples may 

 indicate the source area of the material (McMaster, 1954; Giles and Pilkey, 

 1965; Judge, 1970), and thus may be used as a natural tracer. Such heavy 

 minerals may form black or reddish concentrations at the base of dune scarps, 

 along the berm, and around inlets. Occasionally, heavy minerals occur in 

 concentrations great enough to justify mining them as a metal ore (Everts, 

 1971; Martens, 1928). Table 4-3 from Pettijohn (1957, p. 117) lists the 26 

 most common minerals found in beach sands. 



Sand is by far the most important littoral material in coastal engineering 

 design. However, in some localities, such as New England, Oregon, Washington, 

 and countries bordering on the North Sea, gravel and shingle are locally 

 important. Gravel-sized particles are often rock fragments, (i.e., a mixture 

 of different minerals), whereas sand-sized particles usually consist of single 

 mineral grains. 



3. Cohesive Materials . 



The amount of fine-grained, cohesive materials, such as clay, silt, and 



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